1. Field of the Invention
This invention relates to a transmitting/receiving switch and a portable terminal unit that are used to switch high-frequency signal transmission lines such as a high-frequency circuit in a radio transceiver or a portable terminal unit.
2. Description of the Related Art
Referring to FIG. 5, there is a block diagram of one example of a conventional transmitting/receiving switch. The shown example includes an antenna terminal 501 connected to an antenna ANT, a transmission terminal 502 connected to a transmission circuit Tx, a reception terminal 503 connected to a reception circuit Rx and a control terminal 504 connected to a control circuit CONT that performs transfer switching controls. The transmission terminal 502 is connected to the anode of a first diode 506 through a first capacitor 505. The cathode of the first diode 506 is connected to the antenna terminal 501 through a node 507 and a second capacitor 508. The antenna terminal 501 is also connected to the reception terminal 503 through a second capacitor 508, a node 507, a quarter-wave line 509 and a third capacitor 510. Furthermore, the anode of the first diode 506 is also connected to the control terminal 504 through a coil 511 and a resistance 512. A midpoint between the coil 511 and the resistance 512 is grounded through a fourth capacitor 513. The anode of a second diode 514 is connected to the midpoint between the quarter-wave line 509 and the third capacitor 510, and the cathode of said second diode 514 is grounded.
Referring to the transmitting/receiving switch shown in FIG. 5, in case of connecting the transmission circuit Tx with the antenna ANT, a positive voltage is supplied from the control circuit CONT to the control terminal 504 that turns on the first diode 506 and the second diode 514. At this time, the direct-current component of the positive voltage supplied from the control terminal 504 is blocked by the first through fourth capacitors 505, 508, 510 and 513, and thus the positive voltage is applied to only the partial circuit that includes the first diode 506 and the second diode 514. Said positive voltage drives the first diode 506 and the second diode 514 to the ON state. In this case, the ON state of the first diode 506 lowers the impedance between the transmission terminal 502 and the antenna terminal 501 to establish a connection between them. Meanwhile, the ON state of the second diode 514 brings the quarter-wave line 509 grounded at high frequencies and makes the impedance from the node 507 toward the reception terminal 503 significantly high. As a consequence, an input signal from the transmission terminal 502 is transmitted to the antenna terminal 501 without leaking into the reception terminal 503.
Also referring to the transmitting/receiving switch shown in FIG. 5, in case of connecting the reception circuit Rx and the antenna ANT, a null voltage is supplied from the control circuit CONT to the control terminal 504. Under this condition, the first and second diodes 506 and 514 are driven to the OFF state and the impedance from the node 507 toward the transmission terminal 502 becomes significantly high. The reception terminal 503 and the antenna terminal 501 are connected through the node 507, quarter-wave line 509 and the third capacitor 510. As a consequence, input signals from the antenna terminal 50.1 are transmitted to the reception terminal 503 without leaking into the transmission terminal 502.
As described above, the conventional transmitting/receiving switch performs reception/transmission switching by supplying a positive voltage or null voltage to the first and second diodes 506 and 514. The conventional art transmitting/receiving switch suffers from a problem when high-power transmission signal is imposed on the transmission terminal 502: the transmission signal is distorted due to the non-linearity of the switching element of the first diode 506; harmonic contents are produced in the transmission signal; said harmonic contents are transmitted to the antenna terminal 501 and radiated from the antenna ANT.